Motivated by recent progress in the mass spectroscopy of the elementary reaction of alkali metals and water dispersed in ultracold helium nanodroplets (S. Müller et al., Phys. Rev. Lett., 2009, 102, 183401.), we investigate the properties of pure and mixed Cs clusters and cluster ions, CslHmO0/+n, from a quantum chemical perspective. The presence of Cs atoms requires a careful choice of the methodology, which we have tested for small molecules for which experimental results were available. With the thus selected density functional, pseudopotential and basis set, we compute the geometry, the ionization potentials and the atomization energy, enabling a proper estimate of the energetics of cluster fragmentation upon photoionization. Based upon these calculations, we are able to construct a fragmentation tree that rationalizes the origin of all peaks observed in the experimental mass spectrum. Infrared spectra are computed, and we introduce a simple mixed quantum-classical model that essentially reproduces the cluster geometries.
